1. Field of the Invention
This invention relates to apparatus having means for rotating an object disposed within a vacuum chamber, and for providing services as heating or cooling thereto. The invention is particularly, though not exclusively, concerned with apparatus for the treatment by particulate beams or radiation of a heated, or cooled, continuously rotating sample maintained under ultra high vacuum conditions, and is especially useful in the processing of semiconductor substrates.
2. Description of the Related Art
A common requirement in vacuum systems is to have a facility for rotating an object, which is typically a sample presented for analysis or treatment. In transmitting rotary motion to the interior of a vacuum chamber it is important to maintain the integrity of the vacuum enclosure, which in the example of semiconductor processing apparatus means providing a seal compatible with ultra high vacuum (UHV) conditions, i.e, pressures of 10.sup.-8 Pa (10.sup.-10 mbar) or less. It is also important to minimize any contamination which may be introduced by the material from which the rotary mechanism is fabricated.
Several types of rotary feedthroughs for vacuum systems are known. One type comprises a shaft or tube sealed by a compressible material where it passes through a wall into the vacuum enclosure. Such feedthroughs, which have been described for example in U.S. patent specification No. 3435128, and by S. J. Pearce and S. L. Barker in the Journal of Physics E, 1977, volume 10, pages 1231 to 1232, are particularly suitable for passing electrical current into or out of a vacuum chamber and, while they do allow some rotary displacement, they are not suitable for a continuous rotary drive, especially not for UHV. An improved rotary feedthrough may be achieved by providing a series of seals with pumps disposed to remove gases which leak past or outgas from the seals. Such a differentially pumped rotary seal has been described by A. Pararas et al in the Journal of Vacuum Science and Technology, 1982, volume 21 page 1031, and is UHV compatible but is most suitable for manually rotating a sample between discrete positions for treatment or analysis, rather than supplying continuous rotary motion. U.S. Pat. No. 3,181,873 reports that a differentially pumped rotary feedthrough may be operated continuously, while maintaining a vacuum pressure of 2.7.times.10.sup.-7 Pa (2.7.times.10.sup.-9 mbar), but this is not a satisfactory vacuum for many modern applications, such as semiconductor processing. Another method for sealing a rotary shaft passing into a vacuum chamber is to use a magnetic fluid constrained by magnetic fields as described in U.S. Pat. No. 3,612,549 for example. Such seals are commercially known as Ferrofluidic.TM. seals, and are available from the Ferrofluidics Corporation (USA); an example of the use of this type of seal for sealing a rotary shaft or electrode entrant into a semiconductor processing chamber is described in European Patent Application No. 0108206.
A shaft seal comprising a PTFE grommet has been described by O. Auciello et al in Vacuum, 1978, volume 26, pages 349 to 350, and was used for sealing around a rotatable assembly comprising tubes connected with, and carrying refrigerant to, a specimen holder. Again, this apparatus, though it is UHV compatible, is more suitable for specimen positioning than for prolonged continuous rotary motion. Mechanisms which are particularly advantageous for driving rotating shafts in UHV systems are those which do not have a seal around the shaft at the wall of the vacuum enclosure. Such mechanisms are: (i) magnetically coupled feedthroughs, as described in U.S. Pat. Nos. 3,157,808 and 3,268,750, in which rotary motion is transmitted by coupling between rotatable magnets inside and outside a vacuum enclosure; (ii) the harmonic drive, as described by E. de Haas in Nuclear Instruments and Methods, 1976 volume 137, pages 435 to 439, and A. A. Parry and R. G. Linford in the Journal of Physics E, 1973, volume 6, pages 701 to 702, in which rotary motion is transmitted, via a flexible membrane, between inner and outer rotating members; (iii) `wobbling` or `nutatory` bellows, as described for example by W. C. Turkenburg et al in Nuclear Instruments and Methods, 1975, volume 126, pages 241 to 245, in which a shaft is bent at one end where it is enclosed in a flexible bellows emergent from the wall of the vacuum chamber, and the shaft is rotated by moving the bellows in a circle; and (iv) a motor, having a stator outside and rotor inside a vacuum chamber as described by T. Engel in the Review of Scientific Instruments, 1981, volume 52, pages 301-302.
In semiconductor processing apparatus, disc-shaped substrates are commonly mounted on a substrate holder, which is itself in the form of a disc and is rotatable about an axis through its centre. One substrate may be mounted on such a disc, with the centers of the substrate and substrate holder being coincident and lying on the rotation axis, or a plurality of substrates may be mounted around a supporting disc at uniform angular separation. Means are generally provided to heat or cool the substrate material, it being particularly important to obtain a uniform temperature distribution across each substrate. The means for rotating the substrate holder must be disposed so as not to interfere with the desired positioning of heat sources or heat sinks, which in known apparatus effectively requires the rotary drive to be positioned off the axis of rotation. The substrate holder is driven via gears from a primary shaft rotating about an axis displaced from the axis of rotation of the substrate holder; the gears may transfer the axis of rotation through 90.degree.. While such systems conveniently allow positioning of services near to a substrate holder they have disadvantages, notably: (i) complexity, and (ii) material released by wear from the gears may contaminate the substrates; it is especially important, for the performance of semiconductor devices, to minimize contamination in the manufacturing process.